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Equine Grass Sickness By Emily F. Haggett BVSc DipACVIM MRCVS


Equine Grass Sickness (EGS) is a devastating disease that affects horses, ponies and donkeys. It is usually fatal. The disease causes damage to the part of the nervous system that controls involuntary functions, including breathing and digestion. The most severely affected body system is the gastro-intestinal tract, producing clinical signs of intestinal paralysis of varying degree. However, damage to more distant nerves also occurs.


What causes Grass Sickness?


Despite extensive and ongoing research, the exact cause of EGS is still not completely understood. Multiple causes have been considered including ingestion of clostridial toxins from soil and mycotoxins produced by fungi. The Animal Health Trust recently completed a large-scale vaccine trial investigating whether vaccination against Clostridium botulinum, type C could help prevent EGS. Unfortunately, the vaccine failed to reduce the incidence of EGS and further investigation is still needed.


Why do some horses and not others get Grass Sickness?


EGS occurs sporadically and is more common in certain geographic areas. The disease is most prevalent in Scotland but has been reported in all areas of the UK. The disease affects only horses that have access to grazing and occurs most commonly aſter horses have been exposed to a new pasture. Young horses (2-7 years of age) are most commonly affected and the peak incidence is in the spring and early summer.


Other risk factors include recent soil disturbances, sandy or loam soil, soil with high nitrogen content, properties with high concentrations of horses, lack of supplemental feeding to horses at grass and a history of prior cases on the premises.


What are the clinical signs?


EGS can be classified into three broad types, based on the clinical signs and length of survival of the affected animal:


Acute – the most severe, with sudden onset of the disease and survival normally only 1-2 days Subacute – similar to acute, usually less severe, but with survival of only 2-7 days Chronic – more gradual onset, with survival of more than 7 days


In cases of acute grass sickness, the horse is suddenly affected by complete gut paralysis. Colic is the main clinical sign and the horse will oſten roll, sweat and paw at the ground. When the horse is examined by a vet, the heart rate is very high and distension of the intestines is detected. Other signs include an inability to eat and the horse may reflux food from its nose. Horses will also experience muscle tremors, especially over the shoulders and flanks. The eyelids are frequently ‘droopy’, giving the horse a sleepy-looking appearance.


The subacute and chronic forms are less sudden in onset. Rapid and severe weight loss and weakness are common, and signs of colic are milder. The horse oſten appears very ‘tucked up’ in


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appearance. Patchy or generalised sweating and muscle tremors occur. Eating is difficult due to paralysis of the muscles involved in swallowing and salivation occurs. In horses that are affected by the slowest form of the disease, inflammation of the nostrils leads to a ‘snoring’ noise whilst breathing.


Dramatic weight loss and a tucked-up appearance in a horse with chronic grass sickness. Image: Rossdales LLP


Diagnosis


All horses with signs of colic should be examined by a vet. In the acute form of the disease, it is oſten very difficult to distinguish between EGS and other types of colic and horses are frequently referred to an equine hospital. Although the disease may be suspected based on the clinical signs the horse is showing, confirmation of the disease is oſten more challenging.


Diagnosis requires examination of a small biopsy of the intestine which is taken during surgery. In some cases, this can be done whilst the horse is standing and sedated, but oſten the procedure is carried out under a general anaesthetic. However, this is usually only performed in horses with acute illness as it can worsen the clinical signs of a horse with more chronic disease.


Ileal biopsy performed whilst the horse is standing and under sedation. Image: Rossdales LLP


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